1. Field of the Invention
The invention relates to an adjustable resonator filter, which has a casing made up of a bottom, walls and a lid, which casing functions as a ground for a transmission path, which is divided with conductive partitions into resonator cavities, and on the transmission path of the resonator filter there is a coupling opening in the partitions separating subsequent cavities. The invention further relates to a method for adjusting the coupling between resonator cavities.
2. Background of the Prior Art and Related Information
Cavity resonators are generally used in communications networks for making filters, especially when the effect of the signal to be transferred is relatively large. This is due to the fact that losses caused by such resonator filters are small, which means only a slight attenuation of the efficiency signal. Additionally their response characteristics are easy to control and adjust even according to strict specifications.
Common radio-frequency resonators are various cavity and coaxial resonators, because they can be used to build filters, which have small losses and withstand relatively large capacities. The basic structure of the resonator comprises an internal conductor, an external conductor comprising of the side walls, a bottom and a lid. The bottom and the lid are in galvanic contact with the external conductor, and all three together form a closed resonator casing. Usually the lower end of the internal conductor is galvanically connected to the bottom and the upper end is in the air, whereby the transmission path formed by the resonator is short-circuited in its lower end and open in its upper end. The filter usually has two or more resonators coupled together.
In most filters, both the location and width of the pass band of the filter is meant to be fixed. In some filters the width of the pass band of the filter is meant to be constant, but the location of the pass band must be possible to select from within a certain total area. Thus an adjustment possibility for moving the pass band is needed in the filter in addition to the basic rigging. In order for the frequency response of the band pass filter to be according to requirements, the pass band thereof must on the one hand be in the right place on the frequency axis and on the other hand have the right width. In a resonator filter this requires that, on the one hand, the specific frequency of each resonator is correct and, on the other hand, the strength of the couplings between resonators is correct.
The width of the pass band is adjusted by altering the strength of the coupling between the resonators and at the inlet and outlet of the filter. The adjustment of the band width is based on arranging a so-called cross coupling between subsequent resonators with the same specific frequency, whereby they have a double resonance. When the coupling is thus further strengthened, the resonance peaks of the double resonance draw away from each other, which naturally has a widening effect on the band. At the production stage the band pass filter is in principle dimensioned so that the strength of the coupling between the middle resonators is the smallest and when moving from the middle toward the ends of the filter the strength of the coupling between the resonators grows. When all couplings are strengthened evenly, the band of the filter widens while the attenuation variation remains small in the pass band.
The adjustment of the strength of the coupling, or shorter the adjustment of the coupling, can be realized in several ways. One way is to equip the structure with metallic screws, so that these extend through the lid of the filter to the coupling openings between the resonators. When turning such a screw for example deeper into the coupling opening, the coupling between the resonators in question grows, which has a widening effect on the band. A disadvantage with using tuning screws is that the boundary surface between them and the surrounding metal can cause harmful passive intermodulation when using the filter. Additionally the electric contact in the threads can deteriorate over time, which causes changes in the tuning and an increase in losses in the resonator.
The coupling between two resonators can also be adjusted by means of a bendable tuning member arranged close to the coupling opening. A disadvantage with such a solution is that in a multi-resonator filter the tuning members may need to be bent in several stages to achieve the desired frequency response. The lid of the filter must be opened and closed with each adjustment, wherefore the tuning takes time and is relatively expensive.
Patent publication U.S. Pat. No. 5,805,033 presents a known manner of adjusting the coupling between resonators of a filter. The filter has a casing made up of a bottom, outer walls and a lid, the space of which casing is divided with conductive partitions into resonator cavities. In order to realize the coupling between the resonators there is an opening in their partition, which extends from the lid to the bottom and which narrows toward the bottom. There is a tuning member in the coupling opening for adjusting the coupling, which tuning member is a circular metal plate parallel to the lid. This is attached to the lid by means of a screw bar extending outside of the filter casing. The above-mentioned problems arise in such an arrangement. With such an arrangement the adjustment area does not necessarily turn out linear.